AAAS (American Association for the Advancement of Science) has a great site full of resources for science, ScienceNetLinks.
ScienceNetLinks has collections of lesson plans, searchable by search term, topic, and grade level. There are also Tools, also searchable, that are hands-on activities, interactivities, teaching aids, and websites along with tips to use them in the classroom.
It also has some great after-school activities to share with students, parents or to use with an afterschool program. There is also a section on Science News.
This is a great resource for anyone teaching science, from grades K - 12.